The goal of this project is to develop a broadly applicable method for convenient cell-type specific expression of transgenes, such as optogenetic reagents, in rat, primate and other mammalian brains. Circuits in the brain consist of diverse neuronal subtypes which are often defined by the selective expression of specific genes. For example, neurons defined by the expression of the D1R and D2R dopamine receptors represent distinct neuronal subpopulations which play distinct roles in reinforcement learning and addiction. The ability to target the expression of a growing number of molecular tools, including flourophores and optogenetic reagents, to such genetically-defined neuronal subtypes provides a powerful method for dissecting neural circuits. However, current approaches for achieving cell-type specific transgene expression typically rely on transgenic mouse lines. Cell-specific transgene expression cannot therefore be easily achieved in other model organisms, such as rats or monkeys, which may offer experimental advantages. Moreover, even when the mouse is a suitable model system, the generation and maintenance of new transgenic mouse lines is expensive, labor-intensive and slow. A general method for achieving specific cell-type expression of transgenes would therefore have many potential applications for research and therapy. We propose to develop a novel strategy for the expression of reporter genes in specific neuronal subtypes. Our strategy is based on the specific expression of either Cre or Flp recombinase in specific neuronal cell-types. The expression of transgenes can then be restricted to neurons that express the recombinase. However, unlike previous strategies in which cell-type specific recombinase expression is achieved through the generation of transgenic knock-in mice, our method can deliver both the recombinase and the transgene via recombinant viruses. Thus our method can be applied in organisms in which the potential for genetic manipulation is limited. Our approach will contribute to the understanding of the brain circuitry underlying addiction, and may eventually allow targeting of neuronal sub circuits in the treatment of human neuropsychiatric disorders.